The present invention relates to a method of and apparatus for abrasive machining of parts, for example of wafers.
Semiconductor circuit manufacturers require two qualities of crystalline silicon wafers: “prime” and “test”. Prime quality wafers are used in construction of semiconductors products, while test quality wafers are used to pre-qualify manufacturing processes. Further, two types of test wafers exist: prime test wafers with higher quality and reclaim-test wafers with standard quality. For example a candidate reclaim-test or simply “reclaim” wafer may consist of a silicon substrate with semiconductor components implanted and/or diffused into one wafer surface. Reclaiming than involves removing the layers and portions of the underlying silicon, which have been implanted or diffused.
Machining is a part of the wafer manufacture or “wafering”. Both general and specific machining processes are used for prime, prime-test and reclaim wafers. The objective of prime and reclaim wafering is to remove as little silicon as possible, while producing as little (SSD) subsurface damage) as possible. The amount of SSD directly corresponds to the time and amount of stock removal of silicon in a chemical-mechanical polishing “CMP” operation. CMP is the most expensive process of all the operations in wafer manufacturing. The less silicon removed (by lapping/etching/polishing or chemical stripping) the more times a customer can reclaim the same wafer and re-use it. Chemical stripping is the most hazardous operation in the reclaim wafering.
During machining of prime wafers a substantial layer of silicon is removed since it is necessary to obtain parallel location of its sides which is equal or less than one micron. After slicing, the non-parallelism is substantial and therefore a lapping process is utilized which provides the parallelism within given limits by removal of a substantial layer of silicon. CMP (polishing) removes then a significant layer of silicon since the subsurface damage reaches a high value after the lapping process. Here there is a contradiction since CMP itself can provide the required TTV (total thickness variation).
During a reclaiming process in order to remove films a chemical stripping is utilized which is very hazardous, and/or lapping processes are used. Both processes remove up to 50 μm of silicon in order to remove 2–5 μm of films. With the use of these processes, the operator does not see the films which are being removed and therefore the layer to be removed is substantially greater than the thickness of the film, so as not to again machine the same. Moreover, SSD is very high. As a result the removed layer of silicon reaches 50 μm.
Magnetic-abrasive machining is known in the art. One of the methods of magnetic-abrasive machining is disclosed in our U.S. Pat. No. 6,146,245. This method however has the disadvantage in that the machining zone is closed due to the fact that two permanent magnets are used which are located at opposite sides of a workpiece, and also there is a weak magnetic field and gradient between the poles, since with such a magnetic field the pressing and retaining forces which act on the grains of the magnetic abrasive powder between two magnets are smaller than in the case of the use of one magnet.
U.S. Pat. No. 5,855,735 discloses a process for recovering substrates. It has the disadvantages of including microfractures in the surface, which require removal of them from the surface. Edge materials are removed by abrasive tape. Wafer thickness reduction during recycling is 30 microns and less per cycle. These processes use chemicals and abrasive slurry, which are environmentally not friendly and require expensive recovery.
U.S. Pat. No. 4,821,466 discloses a method of grinding a workpiece, in which the magnets are located at one side of the workpiece. However, in this case also the machining zone is closed by the workpiece which is held in the device and faces toward the magnets. Abrasive grains are located between the surface to be machined and the magnets. The magnets include a group of magnets placed side by side, in which adjoined poles have different polarity with respect to one another.
U.S. Pat. Nos. 5,449,313; 5,616,066 and 5,577,948 disclose further methods of polishing of an object. In these methods the surface of the workpiece to be machined is also closed by the workpiece which is held in a device and a magnet, despite the fact that the magnets are located at one side of the workpiece.
U.S. Pat. No. 5,419,735 discloses a magnetic barrel finishing machine which has a rotary disc made of a non ferromagnetic material, a plurality of permanent magnets radially mounted and irregularly arranged on the rotary disk, and magnets located at the one side of a part to be machined. The axis of the rotary magnets coincide with the axis of one immovable container. In this reference the plurality of permanent magnets provide magnetic lines of force which acts in a circumferential direction of the rotary disk and in a radial direction of the rotary disk inwardly or outwardly. Such different motions or flows of the work pieces and abrasive media do not provide a mutually perpendicular pattern of scratches. It is also believed that the machine disclosed in this reference does not operate in practice as described, since different polarity of the magnets and their arrangement does not generate an alternating magnetic field that cause the workpieces and the abrasive medium to flow in an irregular fashion in two different groups. In fact, when a mixture of abrasive medium and workpieces is located in the container, a magnetic component of abrasive medium and/or workpieces is pressed to the bottom of the container after the locations where the permanent magnets are arranged. The polarity of the magnets and their arrangement does not influence this. This is done by those elements which can be attracted or pressed to the magnets. A part of them is not pressed directly against the bottom, but is pressed to non magnetic elements which lien the bottom. If the magnetic elements contain a magnetic abrasive, then an abrasive cutting takes place. If they do not contain a magnetic abrasive, or in other words the abrasive component is not magnetic, then the abrasive cutting does not take place since the abrasive elements are rotated together with the non magnetic elements.
It is believed that the magnetic-abrasive machining, in particular of wafers can be further improved.